1. Field of the Invention
The present invention relates generally to electronic circuit assemblies. More particularly, the present invention relates to electronic circuit assemblies having surface mount devices with anti-tombstoning solder joints.
2. Disclosure Information
A typical surface mount device (SMD) is mechanically and electrically attached to a printed circuit board (PCB) substrate by providing mounting pads on the substrate, depositing solder paste onto these pads, placing the SMD on the solder paste/mounting pads such that its terminations are aligned with their respective mounting pads, reflowing the solder through the application of heat, and allowing the solder to cool so as to form a solder joint connecting each termination with its respective mounting pad.
Once the solder paste deposition atop each mounting pad melts, pressure forces and surface tension forces from the molten solder begin acting at the interface between the solder joint outer surface and the SMD, thereby exerting forces and moments upon the SMD. These forces are illustrated in FIG. 1, where P.sub.1 and P.sub.2 represent the pressure (suction) forces and F.sub.1 and F.sub.2 represent the surface tension forces acting by the solder joint upon the SMD, and where W represents the weight of the device. It is well known that during the reflow process the solder depositions melt at slightly different times, such that solder joints at one end of the SMD may melt and begin exerting forces on the SMD before solder joints at another end. This uneven application of forces often causes the SMD to undergo two related phenomena known as "tombstoning" and "decentering".
Tombstoning occurs when a net moment is exerted on the SMD by melted solder joints at one end of the device such that the device rotates--first about the device's center of mass, and then about a fulcrum point G, located at the outer edge of the bottom termination adjacent the molten solder joint, when the fulcrum point touches down upon the top surface of the adjacent mounting pad--whereupon the SMD "stands up" on that end. This causes the device to detach from the unmelted or partially melted solder paste at the opposite end. Decentering occurs when a net force is exerted on the device in a direction toward the molten solder joint, thus tending to pull the opposite end of the device off of its unmelted or partially melted solder paste. Device decentering typically occurs before tombstoning does, and it exacerbates the tombstoning problem by detrimentally changing the angles made by F.sub.1 and F.sub.2 with their respective lever arms. Both tombstoning and decentering can cause the device to be mechanically and electrically disconnected from the mounting pads on the end where the solder paste melted last. As indicated in FIG. 1, forces P.sub.1 and F.sub.1 contribute toward tombstoning and decentering.
Prior art attempts at addressing the problems of tombstoning and decentering have focused on making the various depositions of solder paste on a PCB melt more simultaneously. However, this is extremely difficult to accomplish with conventional reflow soldering equipment and processes. It is desirable, therefore, to provide a way of reducing tombstoning and decentering which does not rely on making the solder depositions melt more simultaneously.